In high voltage apparatuses such as high voltage transformers, it is common that the insulation system comprises a fiber based insulation impregnated with an insulating liquid, for example a cellulose based insulation drenched in oil or other dielectric liquid, or insulation based on a thermoplastic polymer, such as polyethylene, with low or essentially closed porosity.
Cellulose based pressboard insulation barriers are normally used in the design of oil filled high voltage (HV) transformers. The purpose of the barriers is to prevent pre-breakdown events such as PD (partial discharge) and electric discharges that may occur on the surface and propagate along the insulation, so called streamers, to develop into breakdown of the insulation system. Normally, a pressboard barrier is designed to be thick enough to prevent an impacting streamer from puncturing the barrier. However, a streamer that does not puncture the barrier may propagate along its surface. A streamer can propagate with different speeds, as a slow mode streamer (2-3 km/s) or a fast event (more than 10 km/s, up to several hundred km/s). For typical transformer designs, a fast propagating streamer along a surface is much more risky than a slow propagating streamer. An example of this is if a transformer is exposed to a lightning impulse of high voltage. Since the duration of the pulse is short (in the order of 50 is) the speed of the propagation strongly affects the likelihood for a full breakdown.
Furthermore, it is known that a fast event occurs for lower voltages along a pressboard surface, compared to what happens in pure oil, see enclosed FIG. 1 (from the article by Lundgaard et al identified below). In this figure is seen how a fast event for the combination oil+pressboard occurs at approximately 250 kV, while the fast event of oil alone does not occur until approximately 350 kV. The velocity of the fast event in oil+pressboard is also much higher. One possible explanation can be that image charges, of the charges in the streamer tip, occur in the pressboard. The force Fimage from the image charges modifies the streamer shape as the streamer is forced down towards the surface of the pressboard, leading to an enhanced field at the streamer tip, which might promote a transition to fast event. See enclosed FIG. 2 (from the article by Lundgaard et al identified below). This explanation is described in the article “Propagation of Positive and Negative Streamers in Oil with and without Pressboard Interfaces”, by Lundgaard, Linhjell, Berg, Sigmond, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 5 No. 3, Jun. 1998.
From the above it is clear that it would be desirable to find means to suppress the onset of fast event along a pressboard surface.
As additional literature explaining how a streamer propagates in oil, it is hereby referred to “A Model for the Initiation and Propagation of Electrical Streamers in Transformer Oil and Transformer Oil Based Nanofluids”, Francis O'Sullivan, PhD Thesis, Massachusetts Institute of Technology, USA, May 2007.